1. Field of the Invention
This invention relates to a singing preventive system and, more particularly, to a system for preventing singing which is caused by an impedance fluctuation on the two-wire side of a hybrid circuit for two-wire to four-wire conversion in a switching system.
2. Description of the Prior Art
In ordinary telephone circuits, there is widely employed a speech path which forms a loop circuit by inserting two one-direction amplifiers into both transmission and receiving paths on the four-wire side of a hybrid circuit for the two-wire to four-wire conversion, and which connects a subscriber line on two-wire. But, when the impedance side on the two-wire side extremely fluctuates due to condition of the subscriber line, leak from transmission path to receiving path occurs and is amplified to cause singing. FIG. 1 shows an example of such a speech path circuit. That is, two-wire circuits 111 and 114 respectively forming two-wire trunks or line circuits are converted by hybrid circuits (HYB) 1.sub.1 and 1.sub.2 to four-wire circuits forming a transmission path 112 and a receiving path 113, respectively, which provide transmission lines to subscribers via amplifiers (AMP) 3.sub.1, and 3.sub.2, respectively. The hybrid circuit (HYB) 1.sub.1 and 1.sub.2 are balanced by balancing networks (BN) 2.sub.1 and 2.sub.2, respectively. It is well-known in the art that, when decrease of return loss of the hybrid circuits (HYB) 1.sub.1 and 1.sub.2 is caused by the impedance fluctuation on the two-wire sides, a leak signal is generated and amplified in the loop circuit to generate a singing phenomena, which degrades the speech quality. A method that has been employed to prevent such singing phenomena is to provide attenuators (PAD) 4.sub.1, 5.sub.1 and 4.sub.2, 5.sub.2 on the two-wire sides and the balancing network sides of the hybrid circuits 1.sub.1 and 1.sub.2 for suppressing decrease of return loss, respectively. But this method has the defect that the attenuators are appreciably troublesome to design and hence are expensive.
In the case of applying a recent digital switching system to such a telephone circuit as mentioned above, analog signals sent to the hybrid circuit (HYB) 1.sub.1 and 1.sub.2 are converted into digital signal by A-D converters and regenerated by D-A converters which are provided in both the transmission and receiving paths 112 and 113. Also in this case, however, singing similarly occurs.